1. Field of the Invention
The present invention relates generally to an automotive turbocharger and more specifically to an control method (and apparatus) therefor by which optimal performance characteristics can be obtained.
2. Description of the Prior Art
For the purpose of simultaneously promoting both fuel economy and power output, automotive internal combustion engine turbochargers are required to operate over a wide range of engine operating conditions and to produce usable amounts of torque even at low and medium engine speeds. In order to achieve this, it is desirable to design the volute passage formed in the scroll housing to have a configuration which will direct the exhaust gases from the engine to the turbine impeller of the turbocharger so that at low engine speeds the rotational speed of the impeller can be increased rapidly while not inducing high exhaust back pressures during high speed engine operation. However, such a compromise cannot in actual practice be produced. Thus it has been necessary to utilize a small capacity turbine and by-pass exhaust gases around the turbine when there is excessive back pressure in the exhaust manifold of the engine and/or when the turbine impeller speed increases to the point of producing an excessive supercharging pressure in the induction manifold.
However, the use of so called "waste gates" for by-passing the exhaust gases is not desirable from the view point of exhaust gas energy recovery. Thus, it has been proposed to effectively vary the capacity of the turbine by utilizing an arrangement such as illustrated in FIGS. 1 and 2, of the drawings and which is disclosed in detail in Japanese Utility Model Application First Provisional Publication Sho No. 53-50310.
In the above mentioned arrangement, the cross sectional area of the inlet portion or nozzle 1 through which the exhaust gases are introduced into the convolute passage 2 defined within the scroll housing 3, is variable via the use of band-like throttling members and actuator arrangement.
In this arrangement, a first band member 4 is pivotally mounted at its downstream end on the inner wall of the scroll housing 3 while a second member 5 is pivotally connected to the upstream end of the first member 4 through a linkage arrangement 6. The upstream end of the second band-like member 5 is pivotally supported on the scroll housing near the attachement flange 7 thereof. An actuator 8 is operatively connected with the linkage arragement 6 for moving the band members from a home position toward and/or into one wherein they throttle the inlet port 1 of the scroll housing 3 in a manner to increase the velocity of the exhaust gases flowing toward the turbine (not shown).
FIG. 2 shows, in block diagram form, a control arrangement used in conjunction with the above disclosed device. This control arrangement controls actuator 8 in response to a signal generated by a potentiometer 9 which detects the position of an accelerator lever 10, and a voltage signal output by a F/V converter 11 which converts an engine speed signal generated by an engine speed sensor 12 into a corresponding voltage. As shown, these two signals are applied to a function generator 13 which outputs a suitable control signal to the actuator via amplifier 14.
This arrangement however, suffers from a drawback in that the temperature of the air being charged into the engine by the compressor of the turbocharger is not taken into account, and so, even though the same amount of air is being charged into the cylinders, the pressure thereof often tends to become excessive, leading to smoke formation in diesel engines and engine damage in the case of Ottocycle (gasoline) engines.
Accordingly, this arrangement, in order to positively safeguard the engine against excessive supercharging pressures, is arranged so that the actuator moves the throttling arrangement between a fully open position OL (see FIGS. 3 and 4) and a maximum throttling position CL which is selected to provide an adequate safety margin. Thus, when the accelerator lever 10 is moved from position R.sub.1 to R.sub.2 for example, the control characteristics provided by the above described control arrangement shift from control schedule "a" to schedule "b" as shown in FIG. 3.
Thus, this arrangement encounters the further drawback in that, as shown in FIG. 3, when the accelerator lever is moved in a manner to accelerate the engine, which for the sake of explanation is assumed to be operating in a non-transitory state with the throttling member fully retracted at position "A" (see FIG. 4), because the engine speed does not change immediately, the control arrangement tends to move the throttling member toward a position B' wherein it fully throttles the inlet port. However, because the actuator cannot move the throttling arrangement beyond the CL limit, the optiminal acceleration characteristics of the turbocharger turbine in the speed range B - C cannot be realized.
A factor affecting the selection of the above mentioned CL limit is that during acceleration, the load on the engine (viz., the amount of air inducted) is markedly increased, leading to knocking if the pressure and temperature of the air charged into the cylinders is excessive.
A further drawback encountered with the above mentioned system is that during partial load the charging characteristics tend to decrease fuel efficiency.